As I understand, barter as a system of exchange tends to develop where the concept of money has been introduced, but physical currency is absent; money makes it relatively easy to compare the value of unlike objects. It also makes it easy to keep track of debts, so an exchange of items of unequal value is not that great a problem. Assuming exchanges are local, that the participants are known to each other, and expect to have ongoing social relationships, unequal exchages are acceptable, even expected; the debt need not be resolved immediately, and in fact debt relationships are social relationships. So you don't necessarily need physical currency for local transactions.Speaking of a little familiarity damaging handwaving... that strikes me as implausible. When people have trade frequently enough to even talk about a market, they invent money. Some stable high-value commodity if no authority is providing something better. They do this even *despite* the local authorities, like cigarette 'money' in prisons. 'Barter' but with a standardized commodity of value.
If things are really bad and you barely have trade, then you don't really have market interaction. Even then, or especially then, I would guess most households would take either food or e.g. metal tools, if some passing adventurer wants to buy something the household can actually spare.
However, it's another matter when you've got exchanges between strangers, who do not expect an ongoing social relationship. Then there's considerably more reason to insist that exchanges be perfectly equal, and resolved at the moment of agreement. In that circumstance, currency with agreed upon value is very useful. And that's where it's going to be a challenge for adventurers to make exchanges.